Infrared Focusing Correction for Mamiyia TLR

Does anyone have a correction chart or formula for infrared focusing with mamiya TLR lenses? I've started working with Konica 120 film and want to use my old mamiya. The c33 body I have has no connection marks on the focusing scales. I thought I would see if any one already has worked this out. I have the 80 and 105 lenses. A quick and dirty alternative is to just construct a correction chart based on the marks on my pentax 105 lens and a 35mm zoom lens set at 80mm.

The amount of correction will depend on the infra-red sensitivity of the film, and on how much visible light you include (i.e. are you using an infra-red filter?). The more visible light you include in the exposure the more conservative you have to be with depth of field, as you have a range of wavelengths to focus.

 

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There's a note in Barry Thornton's 'Elements' that when he tried Konica 750 the first time he tended to over-correct.

 

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I thought Konica provided notes on focus adjustment with the film?

 

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I have a suspicion that no two lens designs/models are identical in their infra-red behaviour if they are designed for visible light use. Thus a general formula could be a problem. Is there an optical designer in the house?

When I first shot the Konica IR, I used a Hassy with 50mm, medium red filter, and corrected the focus to the IR mark on the lens. My shots of Mt. St. Michel taken at a distance of say 1/2 - 3/4 mile were a little off focus. I was using a tripod.

 

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Since, I have heard that there is little correction needed for this film and the Hassy lenses and since then, I have experimented with it with filters from a dark red B&W up to the visibly opaque, with a Mamiya 645 zoom and close up, and also with a Pentacon Six with 30mm fisheye and orange filter. My best results (focus wise) so far have been with no adjustment what so ever, and I would like to hear from anyone else.

 

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BTW, I used a rather mild russian orange filter with the P 6 lens because the filters are rear mount, and I could not get a filter cut to fit the rear of the lens, so I was stuck with it, the results were definately IR, with white leaves, dark sky, and black water shot in the summer in Paris along the Seine.

 

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I do get more extreme IR effects with the darker filters however.

 

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I do have the Mamiya C330 as well, but normally do not shoot it with IR.

 

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Hope this helps you, if anyone has more experience either supporting these results , or countering them, I would be pleased to hear about it.

 

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John

There is a theory that an acceptable correction can be expressed as a percentage of focal length . I recall an article in a trade magazine (aimed at forensic photographers) a while ago about this effect, something along the lines of adding .5% or 1% to the extension. It was accompanied by sample photos using both a standard 105 Micro-Nikkor and the 105 UV-Nikkor. Its probably correct and for super critical (high magnification macro or other limited depth of field) work it's worthwhile, but in my experiences (events & landscape) the error was hidden by the depth of field.

I, too, prefer to use a TLR (Rollie) or a rangefinder camera (Leica) for IR as I find it damn hard to focus through a Wratten 87 filter.